Image pickup devices such as digital cameras and camera phones are widely used to take photographs. As known, the definition of the object taken by the image pickup device is largely effected by the focusing operation of the image pickup device. In order to achieve high image quality of the object, the focal length should be properly adjusted. In other words, the quality of the image pickup device is highly dependent on the auto focus method applied to the digital camera.
Generally, the auto focus methods are classified into two types, i.e. an active auto focus method and a passive auto focus method. Since the active auto focus method needs extra detector and beam projector, the cost of the digital camera is increased. In other words, the passive auto focus method is more popular.
FIG. 1 is a flowchart illustrating a passive auto focus method for use in a portable electronic device according to the prior art. For performing the passive auto focus method, the lens of the image pickup device is moved to different focusing positions by different lens step numbers, and the focus values at different positions are analyzed in order to discriminate whether the image is sharp or blurred. Hereinafter, the passive auto focus method will be illustrated in more details with reference to FIG. 1. First of all, the lens of the portable electronic device is moved to a first position by a lens step number, and then the image of the object corresponding to the first position of the lens is captured (Step S1). Then, the focus value of the image is calculated (Step S2). If this focus value is the maximum focus value (Step S3), the auto focus method is finished. Otherwise, the lens is moved to a next position by a next lens step number (Step S4), and the Step S1, S2 and S3 are repeatedly done until the maximum focus value is searched.
Moreover, a portable electronic device for measuring a distance of an object by performing an auto focus function has been disclosed in the prior art. FIG. 2A is a schematic perspective view illustrating the outward appearance of a portable electronic device for measuring distance according to the prior art. FIG. 2B is a schematic perspective view illustrating the outward appearance of the portable electronic device of FIG. 2A and taken from another viewpoint. As shown in FIGS. 2A and 2B, the portable electronic device 1 is a camera phone. The portable electronic device 1 comprises a display screen 11, an image pickup module 12 and a computing unit 13. The image pickup module 12 is used for capturing an image of an object T (see FIG. 3).
FIG. 3 is a schematic diagram illustrating an object imaged by an image pickup module of a conventional portable electronic device. As shown in FIG. 3, the image pickup module 12 comprises a lens 121, a transmission mechanism 122 and an optical sensing element 123. The transmission mechanism 122 is connected to the lens 121 for moving the lens 121 during the auto focus method is done. The object T is focused by the lens 121, thereby generating an image on the optical sensing element 123. The image pickup module 12 has a focal length f. The distance from the optical sensing element 123 to the lens 121 indicates an image distance q. The distance from the object T to the lens 121 indicates an object distance p. For acquiring the objective distance p, the object T needs to be shot by the image pickup module 12 of the portable electronic device 1 according to an auto focus method. For performing the auto focus method, the transmission mechanism 122 generates motive power to move the lens 121. Until the lens 121 is moved to a position having a maximum focus value, the object T is shot by the image pickup module 12, thereby generating an image I on the optical sensing element 123. Since the focal length f and the image distance q are known, the objective distance p could be obtained according to a Gaussian imaging formula (i.e. 1/p+1/q=1/f). In addition, the information associated with the object distance p could be shown on the display screen 11 to be viewed by the user.
As previously described, the distance of the object from the portable electronic device 1 is acquired by firstly obtaining the image distance q according to an auto focus technology and then using the computing unit 13 to compute according to the Gaussian imaging formula. Since the portable electronic device 1 could measure the object distance p after the computation of the computing unit 13 is completed, the use of the portable electronic device 1 to measure the object distance p is time-consuming. On the other hand, the computation of the computing unit 13 usually results in a resource contention problem of the portable electronic device 1, and thus the performance of the portable electronic device 1 is deteriorated.